Oncogene studies have involved ras encoded proteins. We analyzed chimeras between ras and rap1A, which encodes a ras-like protein that can suppress ras-transformed cells. Chimeras that were discordant with respect to their sensitivity to GTPase acceleration were identified; some were sensitive to ras-GAP but resistant to NF1, and others were sensitive to cytoplasmic rap- GAP but resistant to membrane rap-GAP. Sensitivity of chimeras to ras-GAP and cytoplasmic rap-GAP was mediated by amino acids that are C-terminal to the effector region. In NIH 3T3 cells, chimeras carrying the p21ras effector region and sensitive only to ras-GAP or only to cytoplasmic rap- GAP were poorly transforming. Thus, distinct amino acids of p21ras and p21rap1A mediate sensitivity to each of the proteins with GAP activity, and ras-GAP and cytoplasmic rap-GAP are major negative regulators of p21ras and p21rap1A, respectively, in NIH 3T3 cells. We also identified and characterized the NF1 protein in mammalian cells. This protein is associated with another protein of very high molecular weight. In cell lines derived from malignant schwannomas that arose in patients with NF1 disease (von Recklinghausen's neurofibromatosis), the levels of NF1 protein were found to be very low and the proportion of ras in the active GTP-bound form was high, although there were no mutations in the ras protein. The results are consistent with NF1 being a tumor suppressor gene that negatively regulates ras. They also show that ras proteins can be activated by defective regulation, as well as by mutational activation. We also used ras mutants to study ras in signal transduction. The results suggest that in NIH 3T3 cells the increase in GTP-bound ras protein in response to serum and growth factors may be mediated mainly by stimulated guanine nucleotide exchange. A region on the ras protein has been identified that may be required for the interaction between ras and exchange factors.